Add Db2 acceptance tests to the build

This adds the Db2 tests to the gradle build (after #286 we have Db2 server on the test VM).

This has been tested using JDBC driver version 4.17.29 downloaded from [IBM Fix Central](https://www-304.ibm.com/support/docview.wss?uid=swg21363866): fix pack: DSClients--jdbc_sqlj-10.5.0.3-FP003.

I tested that locally via #295 (copy db2jcc4.jar to libs as `db2jcc4-4.17.29.jar`).

- [ ] Seems [docs](https://github.com/dbfit/dbfit/blob/master/website/docs/database-specific-information.md#db2) should also be updated regarding the db2 driver JAR - Type 4 driver has **4** suffux.

resolves #294

/cc @MMatten